The invention relates to a process for preparing polyacrylates having an average molecular weight of between 250,000 g/mol and 1,000,000 g/mol and a narrow molecular weight distribution.
Owing to ongoing technological developments in the coating process, there is a continuing demand for new developments in the field of pressure sensitive adhesives (PSAs). In industry, hotmelt processes with solvent-free coating technology for preparing PSAs are of growing significance, since the environmental strictures are becoming ever greater and the prices of solvents are rising. Hotmelt processes are already state of the art for SIS adhesives. In contrast, acrylic PSAs are still processed largely from solution. In this respect, an excessive average molecular weight continues to present problems, since, although it is essential for high shear strength, it causes a sharp rise in the flow viscosity, so that acrylic hotmelts with an average molecular weight of  greater than 1,000,000 g/mol are difficult to process from the melt.
On the other hand, low molecular weight acrylic hotmelts have already been successfully implemented as hotmelt PSAs (BASF AG, e.g. UV 203 AC resins). Here, benzophenone derivatives or acetophenone derivatives are incorporated as acrylated photoinitiators into the acrylic polymer chain and then crosslinked with UV radiation [U.S. Pat. No. 5,073,611]. Nevertheless, the obtainable shear strength with such systems is still not satisfactory, although as a result of the low average molecular weight (≈250,000 g/mol) the flow viscosity is relatively low.
The preparation of relatively high molecular weight acrylic PSAs (average molecular weight between 250,000 g/mol and 1,000,000 g/mol) requires specific polymerization processes. Polymerization cannot be carried out without solvent, since at a certain point in time the flow viscosity becomes too high and the conversion of the reaction is very low. The residual monomers would disrupt the hotmelt process. Consequently, acrylic monomers are polymerized conventionally in solution and then concentrated in a concentrating extruder [EP 0621 326 B1].
Nevertheless, the concentration of this acrylic PSA causes problems, since for environmental reasons solvent mixtures, such as special boiling point spirits and acetone, for example, are frequently used (state of the art). Toluene is suspected of being carcinogenic and is therefore no longer utilized. A mixture of solvents leads to the absence of a continuous boiling point in the concentration process, with the consequence that it is very difficult to remove the solvent to a fraction of less than 0.5% (percent by weight based on the polymer). Attempts are therefore made to polymerize acrylates in only one solvent and with one regulator. The regulator meets the functions of avoiding gelling, lowering the average molecular weight, absorbing the heat given off in the initiation phase, lowering the molecular weight distribution, and yet ensuring a high conversion.
The regulators used are generally thiols, alcohols or halides, such as carbon tetrabromide, for example [cf., for example, H.-G. Elias, xe2x80x9cMakromolekxc3xclexe2x80x9d, Hxc3xcthig and Wepf Verlag, Basle, 5th Edition, 1990]. The use of halide regulators is decreasing persistently, however, on environmental grounds. Thiols and alcohols are suitable as regulators and, depending on concentration, greatly reduce the average molecular weight of the polymer but lead to a marked broadening of the molecular weight distribution. This is undesirable for acrylic PSAs, since polyacrylates with too low a molecular weight sharply reduce the cohesion and polyacrylates with a very high molecular weight make the melt viscosity a hindering factor in processing as a hotmelt.
In recent years, in contrast, a new polymerization process has been developed which makes it possible to prepare a large number of polymers with a very narrow molecular weight distribution (Macromolecules, 1999, 32, 5457-5459; WO 98/01478). The polymers described therein, however, all have a low average molecular weight ( less than 200,000 g/mol). Moreover, in all cases the conversion is well below 90%. Both the residual monomer fraction and the low average molecular weight rule out use in the hotmelt process and use as PSAs.
It is an object of the invention to provide a process for preparing polyacrylate compositions of sufficiently high average molecular weights to be used as pressure sensitive adhesives, yet retaining the capacity for the processing of the hotmelt process, by achieving high conversion in the polymerization with a narrow molecular weight distribution.
This object is achieved, surprisingly and unforeseeably for the skilled worker, by a process as described in the main claim. The subclaims describe further developments of said process and applications of the polyacrylates prepared by said process.
Claim 1 relates accordingly to a process for preparing polyacrylates wherein the monomer mixture for preparing the polyacrylates is composed of at least 70% by weight of at least one acrylic monomer of the general formula 
where R1 is H or CH3 and R2 is H or an alkyl chain having 1-20 carbon atoms, the monomers are polymerized in the presence of at least one free-radical initiator by free-radical polymerization with at least one thioester as polymerization regulator, and the polymerization is conducted such that the average molecular weight of the polyacrylates is in the range from 250,000 g/mol to 1,000,000 g/mol, and the molecular weight distribution, Mw/Mn, is  less than 4.
As free-radical initiators for the free-radical polymerization it is possible to use any customary initiators known for this purpose for acrylates. The preparation of C-centered radicals is described in Houben Weyl, Methoden der Organischen Chemie, Vol. E 19a, pp. 60-147. These methods may be employed analogously. Examples of radical sources are peroxides, hydroperoxides, and azo compounds; as nonexclusive examples of typical radical initiators mention may be made here of potassium peroxodisulfate, dibenzoyl peroxide, cumin hydroperoxide, cyclohexanone peroxide, di-t-butyl peroxide, azodiisobutyronitrile, cyclohexylsulfonyl acetyl peroxide, diisopropyl percarbonate, t-butyl peroctoate, and benzpinacol. In one very preferred variant, the initiators are added in a number of stages, so that the conversion is increased to more than 90%. The residual monomer content of the polymer can in this way be decreased to below 10% by weight; by virtue of a low residual monomer content, the properties of the polyacrylate are considerably improved in respect of its further processing by the hotmelt process.
The initiators added at the beginning are preferably chosen for their low propensity to form side chains in the polymers; their grafting activity is preferably below a level of xcex5 less than 5 at the temperature of the reaction mixture when the initiator is added.
The absolute grafting activity (crosslink efficiency) is defined as the number of chemical side chains formed per 100 mol units of decomposed initiator. In analogy to van Drumpt and Oosterwijk [Journal of Polymer Science, Polymer Chemistry Edition 14 (1976) 1495-1511], it is possible to specify a value for this number by determining the dimers in a defined solution of the initiator; see also DE 43 40 297 A1:
A precisely 0.1 molar solution of the initiator is decomposed in n-pentadecane under an He atmosphere. The reaction time is chosen to correspond to ten times the half life of the respective initiator at the chosen temperature. This ensures virtually complete decomposition of the initiator. Subsequently, the fraction of dimeric pentadecane produced is measured by means of GLC. The percentage fraction xcex5 is stated as a measure of the grafting activity. The reaction temperature is normally chosen so that the half life of the test initiator at this temperature is 15 minutes.
High xcex5 values for the grafting activity imply a high propensity of the initiator to form side chains in the polymerization, whereas small xcex5 values result in preferentially linear polymers.
In one preferred procedure, the process sequence is as follows:
the reaction solution used is an at least 50% strength solution of the monomers with added initiator(s) and thioester(s),
the free-radical polymerization is conducted within a temperature range from 50xc2x0 C. to 90xc2x0 C.,
during the polymerization the batch is reinitiated at least once using a free-radical polymerization initiator with a low propensity to form side chains (grafting activity xcex5 less than 5 at the prevailing reaction temperature),
if desired, the reaction is controlled by diluting the reaction solution according to the viscosity of the polymer,
controlled reinitiation is carried out with up to 2% by weight, based on the monomer mixture, of an initiator with an increased propensity to form side chains (grafting activity xcex5 greater than 10 at the prevailing reaction temperature), and
the polymerization is conducted to a conversion  greater than 90%, preferably  greater than 95%.
Preferred initiators having a low xcex5 value (xcex5 less than 5) are those whose radicals, owing to their low energy content, cause infrequent, if any, abstraction of hydrogen from the polymer chains. It is preferred here to use, for example, azo initiators such as azoisobutyrodinitrile or derivatives thereof, such as 2,2-azobis(2-methylbutyronitrile) (Vazo67, DuPont).
Initiators having a high side-chain formation propensity (high xcex5value  greater than 10) give high grafting yields even at relatively low temperatures. Particular preference is given here to using bis(4-t-butylcyclohexyl) peroxodicarbonate (Perkadox 16, Akzo Chemie), dibenzoyl peroxide or the like.
The polymerization may be conducted in the presence of an organic solvent or in the presence of water or in mixtures of organic solvents and/or water. As solvents for the polymerization it is possible to use all solvents which are suitable or commonly used for free-radical polymerizations, with acetone, ethyl acetate, petroleum spirit, toluene or any mixtures of these solvents being particularly appropriate.
It is preferred to use as little solvent as possible. Depending on conversion, temperature, and initiation, the polymerization time is between 6 and 48 h.
The pressure sensitively adhesive polyacrylates prepared have an average molecular weight of between 250,000 and 1,000,000 g/mol, said average molecular weight being measured by means of SEC or GPC. The (co)polymers prepared generally possess a lower molecular weight distribution than in the polymerizations conducted analogously using conventional regulators. The polydispersity may be reduced to a level of less than 4. As result of the low molecular weight distribution, there is a reduction in the flow viscosity of the pressure sensitive adhesive, and the hotmelt PSA is significantly easier to process as a hotmelt (lower melting temperature required, higher throughput for concentration).
In one particularly preferred variant of the inventive process, the thioesters used comprise compounds of the following general structural formula: 
where R and Rxe2x80x2 are chosen independently of one another and R is a radical from one of groups i) to iv) and Rxe2x80x2 is a radical from one of groups i) to iii):
i) C1-C18 alkyl, C2-C18 alkenyl, C2-C18 alkynyl, in each case linear or branched; aryl, phenyl, benzyl, aliphatic and aromatic heterocycles,
ii) xe2x80x94NH2, xe2x80x94NHxe2x80x94R1, xe2x80x94NR1R2, xe2x80x94NHxe2x80x94C(O)xe2x80x94R1, xe2x80x94NR1xe2x80x94C(O)xe2x80x94R2, xe2x80x94NHxe2x80x94C(S)xe2x80x94R1xe2x80x94C(S)xe2x80x94R2, 
where R1 and R2 are independently chosen radicals from group i),
iii) xe2x80x94Sxe2x80x94R3, xe2x80x94Sxe2x80x94C(S)xe2x80x94R3,
where R3 is a radical chosen from one of groups i) and ii),
iv) xe2x80x94Oxe2x80x94R3, xe2x80x94Oxe2x80x94C(O)xe2x80x94R3,
where R3is a radical chosen from one of groups i) and ii).
As regulators it is preferred, accordingly, to use dithioesters and trithiocarbonates.
As a result of the process thus chosen, it is possible very effectively to prepare the pressure sensitive adhesives having the desired PSA properties.
In one advantageous embodiment of the inventive process, the monomers used further comprise vinyl compounds with a fraction of up to 30% by weight, in particular one or more vinyl compounds chosen from the following group:
vinyl esters, vinyl halides, vinylidene halides, nitriles of ethylenically unsaturated hydrocarbons.
As examples of such vinyl compound mention may be made here of vinyl acetate, N-vinylformamide, vinylpyridines, acrylamides, acrylic acid, ethyl vinyl ether, vinyl chloride, vinylidene chloride, acrylonitrile, maleic anhydride, and styrene, without wishing this list to represent any unnecessary restriction. It is also possible to use all further vinyl compounds which fall within the group indicated above, and also all other vinyl compounds which do not fall into one of the classes of compound mentioned above.
In one very preferred procedure, the initiators which are added at the beginning to the monomer solution in the inventive process are those with a low propensity to form side chains (grafting activity xcex5 less than 5 at the prevailing temperature of the reaction solution). The initiators are preferably used in a fraction of 0.001-1% by weight, more preferably of 0.05% by weight, based on the monomer mixture.
In a further advantageous variant of the inventive process, the thioester is used with a weight fraction of 0.001%-5%, in particular from 0.025% to 0.25%. In the sense of the invention it is also very advantageous if the molar ratio of free-radical initiator to thioester is in the range from 50:1 to 1:1, in particular between 10:1 and 2:1.
For the use of the polyacrylates prepared by the inventive process as pressure sensitive adhesives, the polyacrylates are optionally optimized by blending with at least one resin. As tackifying resins for addition, it is possible to use, without exception, all tackifier resins which are already known and are described in the literature. Representatives that may be mentioned include pinene resins, indene resins, and rosins, their disproportionated, hydrogenated, polymerized, esterified derivatives and salts, aliphatic and aromatic hydrocarbon resins, terpene resins and terpene-phenolic resins, and also C5 resins, C9 resins, and other hydrocarbon resins. Any desired combinations of these and further resins can be used in order to set the properties of the resultant adhesive in accordance with what is required. In general, it is possible to use any (soluble) resins that are compatible with the corresponding polyacrylate; in particular, mention may be made of all aliphatic, aromatic, and alkylaromatic hydrocarbon resins, hydrocarbon resins based on simple monomers, hydrogenated hydrocarbon resins, functional hydrocarbon resins, and natural resins. Explicit reference is made to the depiction of the state of knowledge in the xe2x80x9cHandbook of Pressure Sensitive Adhesive Technologyxe2x80x9d by Donatas Satas (van Nostrand, 1989).
In a further advantageous development, the PSA is admixed with one or more plasticizers, such as low molecular weight polyacrylates, phthalates, poly(ethylene glycol)s or poly(ethylene glycol)s functionalized by amino groups, or plasticizer resins. In one preferred further development, phosphates/polyphosphates are used for acrylic hotmelts.
The acrylic hotmelts may further be blended with one or more additives such as aging inhibitors, light stabilizers, ozone protectants, fatty acids, resins, nucleators, expandants, compounding agents and/or accelerators.
They may be further be admixed with one or more fillers such as fibers, carbon black, zinc oxide, titanium dioxide, hollow or solid glass (micro)beads, microbeads made from other materials, silica, silicates and chalk, with the addition of nonblocked isocyanates also being possible.
Especially for use as a PSA it is advantageous for the inventive process if the polyacrylate is applied preferentially from solution as a film to a support or to a backing material.
One advantageous further development of the inventive process comprises concentrating the polyacrylates prepared as described above to give a polyacrylate composition whose solvent content is xe2x89xa62% by weight. This procedure is preferably conducted in a concentrating extruder. In one advantageous variant of the process, the polyacrylate composition is then applied as a hotmelt composition in the form of a film to a support or to a backing material.
For the two latter variants of the inventive process, preferred backing materials used, for adhesive tapes for example, are the materials customary and familiar to the skilled worker, such as films (polyester, PET, PE, PP, BOPP, PVC), nonwovens, foams, wovens and woven sheets, and also release paper (glassine, HDPE, LDPE). This list is not conclusive.
For PSA use it is particularly advantageous to crosslink the polyacrylates following coating onto the support or onto the backing material. For this purpose, for the production of PSA tapes, the polymers described above are optionally blended with crosslinkers. Crosslinking may advantageously be induced thermally or by means of high-energy radiation, in the latter case in particular by electron beams or, following the addition of appropriate photoinitiators, by ultraviolet radiation.
Preferred radiation-crosslinking substances according to the inventive process include, for example, difunctional or polyfunctional acrylates or difunctional or polyfunctional urethane acrylates, difunctional or polyfunctional isocyanates or difunctional or polyfunctional epoxides. Very advantageously, it is likewise possible to use metal chelate compounds. However, use may also be made here of any other difunctional or polyfunctional compounds that are familiar to the skilled worker and are capable of crosslinking polyacrylates.
Suitable photoinitiators include preferably Norrish type I and type II cleavers, with some possible examples of both classes being benzophenone derivatives, acetophenone derivatives, benzile derivatives, benzoin derivatives, hydroxyalkylphenone derivatives, phenyl cyclohexyl ketone derivatives, anthraquinone derivatives, thioxanthone derivatives, triazine derivatives or fluorenone derivatives, this list making no claim to completeness.
Also claimed is the use of the polyacrylate prepared by the inventive process as a pressure sensitive adhesive.
Particularly advantageous is the use of the polyacrylate PSA, prepared as described, for an adhesive tape, in which case the polyacrylate PSA may be applied to one or both sides of a support.